1. Field of the Invention
The invention disclosed herein relates generally to ion implantation methods employed in the manufacturing process of semiconductor devices. Specifically, this invention relates to an improved implantation process for manufacturing semiconductor devices that include shallow p-type or n-type regions by delivering ultra low energy (0.2 to 20 keV) ion beams to targets by employing an improved ion implantation method.
2. Description of the Prior Art
As semiconductor device dimensions continue to shrink source-drain junction depths are reduced accordingly. Shallow junction formation is, however, fast becoming one of the major limiting factors in the modern semiconductor fabrication process. To those skilled in the art of maling modern Ultra Large-Scale Integrated (ULSI) circuits conventional ion implantation methods do not provide production worthy solutions to the semiconductor industry.
A technology roadmap presented by Saito in IIT""98 [International Conference on Ion Implantation Technology, Kyoto, Japan, 1998]) indicates that sub-keV implantation energy is required for the 0.15 xcexcm and below technology nodes. For example, 0.5 keV boron ions are used for 0.13 xcexcm devices and 0.2 keV for 0.1 xcexcm devices. Conventional implantation systems are unable to provide production worthy beam currents at energies below 2 keV because of space-charge beam blow up (i.e. divergence) associated with low energy beams.
One method that is used to achieve high beam currents at energies below 2 keV involves extraction of ions at higher energies than that desired, followed by a mass analysis, and then the ions are decelerated just before they reach the targets [J. G. England, et al., U.S. Pat. No. 5,969,366: Ion Implanter With Post Mass Selection Deceleration, 1999]. One problem with this method, however, is that neutralization of ions prior to deceleration may occur in the region between the mass analyzer and the deceleration electrodes when the ions interact with residual gases in the beam line. These resulting neutrals will not be decelerated by the deceleration electric fields and may therefore reach the wafers at higher than desired energies. This effect is known as energy contamination and leads to a deeper than desired dopant depth profile. Energy contamination is only tolerable to a level of xcx9c0.1%, depending on the energy of the neutral fraction, to provide a sufficient margin against shifts in device performance [L. Rubin, and W. Morris, xe2x80x9cEffects of Beam Energy Purity on Junction Depths in Sub-micron Devicesxe2x80x9d, Proceedings of the International Conference on Ion Implantation Technology, 1996, p96].
Reducing the beamline pressure can reduce the energy contamination but this approach requires the chamber pressures to be kept very low (5.0E-7 torr). This level of vacuum is, however, very difficult to be maintained under normal operating conditions due to the out-gassing of the photo-resist coating of patterned devices as well as the contribution from feed gases. Another issue is the variation in the level of contamination. Pressure fluctuations during the implant can cause across wafer effects. Day-to-day changes in residual vacuum or photo-resist quality may cause batch-to-batch effects. There is a potential for the loss of wafers, potentially worth millions of dollars, due to undetected vacuum problems. Methods have been invented to detect energy contamination due to high chamber pressure during ion beam deceleration [B. Adibi, U.S. Pat. No. 5,883,391: Ion Implantation Apparatus And A Method Of Monitoring High Energy Neutral Contamination In An Ion Implantation Process, 1999].
FIG. 1 is a functional block diagram for a conventional low energy ion implantation system used for generating a low energy beam 10 from an ion source 15 for carrying out a low energy ion implant on a target wafer 20. The ion beam 10 generated from the ion source 15 is mass analyzed by a magnetic analyzer 25 and travels along a curved trajectory that makes a nearly ninety-degree turn. The positively charged particles are decelerated by applying a negative voltage 30 along the ion beam path 10 for reducing the implant energy when the ion beam 10 passes through the deceleration optics 35 to reach the target wafer 20. The drawback of this system is the presence of the neutral particles, which are not decelerated by the negative voltage 30. These neutral particles will bombard the target wafer 20 at a higher energy than the decelerated charged particles and cause undesirable effects to the devices. The vacuum has to be maintained at a very high level within the sealed space by the beamline chamber 40 and the target chamber 50 to minimize the neutralization of the ion beam.
The use of plasma electron flood systems and out-gassing of photoresist wafers are two reasons why it is impractical to have a high vacuum in the chambers 40 and 50. To prevent beam blow-up after deceleration and wafer charging during implants, an electron flood source or a plasma flood source should be placed between the deceleration optics 35 and the target wafer 20. These flood sources usually require substantial gas flow, such as xenon or argon gas, for the best performance. The gas flow out of the flood source increases the gas pressure in chambers 40 and 50. Additionally, ion beam bombardment of the target wafer with patterned photoresist coating generates significant out-gassing that also contributes to an increase of the gas pressure in the chambers 40 and 50, particularly near the wafer.
For the above stated reasons, traditional techniques of ion implantation using conventional types of energy deceleration systems as described above do not provide a viable solution for the difficulties currently associated with the fabrication processes employing very low energy implantation. There is a pressing need in the art of IC device fabrication for new systems and methods used for very low energy ion implantation. Specifically, for devices that require shallow p-type and n-type junctions, new methods and systems are required to resolve these difficulties and limitations with effective control over energy contamination of low energy beams.
Separating a decelerated ion beam from neutral particles by electrostatic field has been used in nuclear fusion technology [Hashimoto et al., U.S. Pat. No. 4,480,185: Neutral Beam Injector, 1984]. Similar concept of this technology can be applied to the ion implantation technology to solve energy contamination problem during ion beam deceleration.
It is the object of the present invention to provide a new ion implant method for low energy implantation to form shallow p-type and n-type junctions in semiconductor devices. Specifically, it is the object of the present invention to present a new ion beam steering and deceleration method for decelerating a charged ion beam and for separating the neutralized particle beam from the ion beam. The neutralized beam, which propagates at a higher energy than the decelerated ion beam, is separated and stopped by a neutral-particle-stopping block before reaching the target wafer. In this way, energy contamination as a result of neutralized particles incident to the target with higher than desired energy is resolved.
An ion implantation method is disclosed in this invention that involves an ion beam deceleration optics that includes a beam deceleration means for decelerating the ion beam for producing a low energy ion beam. The beam deceleration optics further includes a beam steering means for generating an electrostatic field for steering the ion beam to a targeted ion-beam direction and separating neutralized particles from the ion beam by allowing the neutralized particles to transmit in a neutralized-particle direction slightly different from the targeted ion-beam direction. The ion beam steering means further includes a beam stopper for blocking said neutralized particles from reaching said target of implantation that minimizes energy contamination from high energy neutralized particles.